In one type of a vehicle, as described in Japanese Kokai Patent Application No. 2002-097939, the vehicle includes an internal combustion engine and a NOx absorber catalyst type of an exhaust gas purification device that collects deposits from the exhaust gas. The deposits include sulfur deposits. When the deposition count of deposits in the exhaust gas purification device exceeds a prescribed value, regeneration or regenerative removal of the deposits is deemed to be required. For regeneration, the temperature of the exhaust gas purification device (NOx adsorption catalyst) is increased to remove the sulfur deposits by burning them off.
This will effectively regenerate (sometimes called “depoison”) the NOx absorber catalyst type exhaust gas purification device.
To avoid deteriorating the NOx absorber catalyst type exhaust gas purification device, it is good to avoid excessive temperature, above the temperature required for burning the sulfur deposits, during regeneration and when the regeneration of the exhaust gas purification device is completed, it is good to reduce the temperature promptly. One way to reduce the temperature of the exhaust gases from the engine is to decrease the air-fuel ratio (to make a correction to bring it to the rich side, that is excess fuel) while the engine is rotating slowly and under a low load. However, attaining a stable temperature may be delayed because the reduction of the temperature is slow when the air-fuel ratio is decreased.